The idea is simple: There are customers who are influenced by an ad but wait to purchase a product later on. So Facebook built a tool to measure it by comparing databases of people who purchased products with those who did or didn't see an ad on Facebook.

The tool is just the latest effort by the world's largest social network to change how advertisers look at the effectiveness of its service and, ultimately, justify charging higher fees for online ads. Facebook is attempting to draw a more direct line between its ads and when a consumer buys a product, even if the person doesn't necessarily click on the link.

Brad Smallwood, who runs marketing science and measurement at Facebook, said his goal is to help advertisers track how ads influence behavior over time.

The way Facebook's tool works is that advertisers send it a database of customers who bought something. That information is randomized and compared to a list of users who did or did not see an ad. The result is data that could show advertising on Facebook helped encourage users to buy a product.

"There is a common refrain in the industry that dollars follow where people are spending their time," said Andrew Lipsman, vice president of marketing and insights at industry researcher ComScore. Ultimately, he added, advertisers want to shift dollars toward ads that are most successful.

Facebook isn't the only company attempting to connect advertising in the digital and real worlds. Others, including market researchers and Internet behemoths alike, have built services hoping to track similar behavior

There are limitations to this type of effort. The biggest hurdle is retailers, whose databases of customers rely on convincing them to offer personal information, like an email address, in the first place. As a result, these databases aren't comprehensive.

Also, there's a danger advertisers could rely too heavily on the data Facebook is offering. Focusing too much on an ad that generates a sale, as opposed to an ad that helps to do something like promote a brand, could be self-defeating.

Consider grocery stores. When the scanner was introduced to store checkout counters 30 years ago, owners learned how well promotions increased sales. So, they focused on them, ComScore's Lipsman said. Today, the industry relies heavily on promotions to draw customers into the store, hurting margins.

For now, Facebook said it's content if advertisers change the way they see its business. The information coming from Facebook's tool, Smallwood said, "will be a better assessment of what's working and what's not." "